3517 datasheet(9/20 Pages) LINER

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LT3517

3517fa

APPLICATIONS INFORMATION

than 1V, the LED current is:

LED

CTRL

SENSE

10 •

(2)

When VCTRL is higher than 1V, the LED current is clamped

to be:

LED

SENSE

100

(3)

The LED current programming feature possibly increases

total dimming range by a factor of ten.

The CTRL pin should not be left open (tie to VREF if not

used). The CTRL pin can also be used in conjunction with

a PTC thermistor to provide overtemperature protection

for the LED load.

For a buck or a buck-boost conﬁguration, the output

voltage is typically level-shifted to a signal with respect

to GND as illustrated in the Figure 4. The output can be

expressed as:

OUT

BE Q

101

•.

()

(5)

49.9k

3517 F02

45.3k

VREF

PTC

CTRL

Figure 2

Setting Output Voltage

For a boost application, the output voltage can be set by

selecting the values of R1 and R2 (see Figure 3) according

to the following equation:

OUT =+

⎛

⎝⎜

⎞

⎠⎟

11 01

•.

(4)

LT3517

VOUT

3517 F03

Figure 3

LT3517

–

LED

ARRAY

VOUT

3517 F04

RSENSE

Figure 4

Inductor Selection

The inductor used with the LT3517 should have a satura-

tion current rating of 2A or greater. For buck mode LED

drivers, the inductor value should be chosen to give a

ripple current “

ΔI” of ~30% to 40% of the LED current.

In the buck mode, the inductor value can be estimated

using the formula:

LµH

µs

BUCK

LED

BUCK

LED

() =

()

•( ) •

–

IIN

(6)

VLED is the voltage across the LED string, VIN is the input

voltage to the converter, and tSW is the switching period.

In the boost conﬁguration, the inductor can be estimated

using the formula:

LµH

µs

BOOST

LED

() =

•( ) •

–

(7)

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